Recent studies suggest the lifetime risk for symptomatic knee OA (KOA) is 45%1. An aging population and increasing rates of obesity contribute to dramatic increases in the prevalence of this condition. Historically, the "disease" of OA is viewed primarily as damage to the cartilage and bone. As such, the magnitude of damage or inflammation of these structures should predict symptoms. Population-based studies suggest otherwise;30- 50% of individuals with moderate to severe radiographic changes of OA are asymptomatic, and approximately 10% with moderate to severe knee pain have normal radiographs2,3. Psychological factors do account for some 4,5 of this variance in pain and other symptoms, but only to a small degree . This failure of peripheral damage, inflammation, or even psychological factors to explain the presence, absence, or severity of chronic pain should not be surprising. To date, no chronic pain state involves a strong relationship between peripheral factors and the level of pain reported. We hypothesize that, although peripheral nociceptive input and to a lesser extent psychological factors are important in leading to pain and symptom expression in KOA, some patients possess varying degrees of non-psychological central nervous system (CNS) factors which play an equally or even more prominent role in the expression of pain and co-morbid symptoms 6-8. Broadly within chronic pain states, subsets of patients have been identified that have prominent CNS mechanisms (as opposed or in addition to, peripheral damage) playing important roles in pain perception. Such factors include diffuse hyperalgesia or allodynia, and/or a lack of endogenous descending analgesic activity 6,8, 9 . The exploration of these CNS factors has been somewhat limited to date in OA, but evidence is emerging that supports the hypothesis that subsets of OA patients do indeed have these mechanisms operative 10-12. Our hypothesis is further strengthened by studies that have identified co-morbid somatic symptoms known to be associated with central pain conditions (e.g., fatigue, sleep problems) to be commonly present in OA 13, 14. Finally, recent RCTs have demonstrated that compounds that alter pain neurotransmitters centrally such as 15,16 serotonin and norepinephrine (e.g., duloxetine, tricyclics) are efficacious in OA . We will identify the role that CNS factors are playing in KOA by first showing that subsets of patients have symptom patterns and experimental sensory testing abnormalities consistent with having a "central" component to their pain. We will then demonstrate the utility of these measures by showing that individuals with KOA with central pain will respond poorly to knee arthroplasty. Such a study possesses 17 high public health impact given the high "failure" rate of knee arthroplasty . If the status quo in practice for KOA is maintained, demand for knee arthroplasty will increase by an expected 700% in the next 20 years 18. Thus, in addition to this study providing a potential paradigm shift in our thinking regarding the "disease" of OA, this study also develops practical clinical tools for identifying the presence of centrally-enhanced pain in OA. PUBLIC HEALTH RELEVANCE: This study aims to examine why individuals with osteroarthritis of the knee respond poorly to knee arthroplasty. With demand for this procedure increasing, and an aging population, it is important to examine why there is such a high failure rate in this population.